1. Field of the Invention
The present invention relates to a mobile range finder for use on a vehicle to detect a relative distance to a desired position in a set monitoring direction, for example, a range finder for detecting the distance between two vehicles.
2. Description of the Related Art
As a prior art of the type described above, there is known a range finder designed to measure the distance between two vehicles by making use of microwaves, ultrasonic waves and the like. However, the prior art has the problem that interference occurs when a vehicle equipped with the same type of range finder comes close to the vehicle concerned, and therefore the use of this type of range finder has heretofore been limited only to detection of the distance between two vehicles which are apart from each other at a considerably short distance, for example, detection of a rearward clearance at the time when the vehicle is being backed up.
Japanese Patent Laid-Open No. 59-197816 (1984) discloses an apparatus which has memory means stored with feature data representing the features of various kinds of vehicle to detect the distance between two vehicles by the following method. Namely, two picture signals representative of the forward view from the vehicle equipped with this apparatus which are obtained by means of two TV cameras are each processed on the basis of feature parameters. Each processed data is compared with the feature data stored in the memory means to detect a vehicle which is ahead of the vehicle concerned, and trigonometrical measurement is carried out on the basis of the spatial offset between the two pieces of processed data concerning the vehicle ahead to thereby detect the distance between the two vehicles.
According to the disclosed apparatus, neither microwaves nor ultrasonic waves are used; therefore, no interference occurs even if the same type of apparatus comes close to it.
The apparatus of the type described above suffers, however, from the following disadvantages. Since vehicle feature data is employed to detect a vehicle which is ahead of the vehicle equipped with the apparatus, it is necessary to store feature data concerning all the existing types of vehicle. Further, every time a new type of vehicle is put on sale, data concerning it must be registered. In other words, it is necessary not only to prepare a memory means having a considerably large capacity but also to renew the contents of the memory means as occasion demands, which is inconvenient and impractical. In addition, to retrieve the feature data stored in the memory means, a great deal of information processing is needed and an excessively long processing time is required.
Separately from the above, there is an idea of detecting the distance between two vehicles by attaching a luminous marker to the rear end of each vehicle, recognizing the marker of a vehicle ahead of the vehicle concerned by reading it with a non-scanning type position sensor that utilizes a lateral photoelectric effect on the surface of a semiconductor, and detecting the distance to the vehicle ahead on the basis of the installation angle of the position sensor to the road surface and the level at which the sensor is installed. In this case, trigonometrical measurement is conducted on the basis of the fact that the installation angle of the position sensor is invariable because it is fixed to the vehicle. However, the vehicle pitches incessantly during traveling due to acceleration and deceleration as well as the conditions of the road surface, causing the installation angle of the position sensor to the road surface to vary incessantly. Therefore, the above-described idea of detecting the distance between two vehicles is inappropriate.